Black Mamba's aren't as aggressive as people think. They will only bite you out of self defense. In fact, most if not all snakes only bite for this reason. They even give you 3 warnings before they bite. They will first raise up and spread their hood. If you don't back away or move closer it will then open its mouth. If you still don't move away or move closer it will do a mock strike, which is rushing at you hissing and suddenly stopping. If you STILL don't move away or go closer it will then bite you. As long as you slowly back away you shouldn't ever get bitten by a Mamba.

They do however move faster then any other snake on open ground. Just 2 drops of venom can kill a human and they can have up to 20 drops in their fangs at a time. It's not the deadliest venom, as someone pointed out to me recently, but it is EXTREMELY fast acting and has been known to kill people within 15 minutes.